Replacing your vehicles exhaust system with a performance exhaust system including a performance muffler can increase your engines horsepower but you'll only usually gain about 10 percent more horsepower from your engine when you change to an aftermarket performance exhaust system and muffler.
Don't expect a huge increase in performance though as you need to do other modifications to the vehicles engine before you'll see a huge increase in engine horsepower and performance.
But an aftermarket performance exhaust system can help some.